4-HPR enhances VP16-mediated cytotoxicity in BM metastatic neuroblasts. A) PTX and BM cells were treated with 20 or 40 μM etoposide/VP16 for 24 hrs. Cell death was measured as previously described and the data are expressed as a percentage of control cells. Mean ± SD of three independent experiments. **P < 0.01 indicates significant difference between PTX and BM cells. B) PTX and BM cells were incubated with 2.5 μM 4-HPR for 96 hrs, then with 20 μM VP16 for 24 hrs. Data points show mean ± SD of a representative experiment. The (*) indicates that synergism was demonstrated for VP16 and 4HPR.